In many applications and primarily in audio reproduction systems, for example in car audios, HI-FI audio systems and similar apparatuses that are intrinsically compact because of stringent installation requirements, as well as in portable apparatuses, power dissipation in final power stages, often quadrupled in order to drive a pair of loudspeakers (front and rear) for each stereo channel, may create heat balance problems. For example, four 20 W amplifiers may have a power dissipation of about 4×12=48 W and because of the limited space available in certain apparatuses, such a relatively large power may be difficult to be dissipated without a significant increase of temperature within the apparatus.
On the other hand, a relatively high temperature of operation may degrade the magnetic tape of cassettes or optical disks (CD), the drives of which are often tightly fitted inside a single apparatus case.
The so-called D-type switching amplifiers are highly efficient and are considered the most appropriate type for these applications.
Unfortunately, switching amplifiers generate electromagnetic emissions that in compact apparatuses interfere with the correct functioning of other systems, reducing their performances. For these reasons, audio signals are frequently amplified using a pair of class AB power amplifiers, operating in single-ended or in bridge configuration depending on the level of the processed signal.
Class AB power amplifiers are less efficient than switching amplifiers and a common technique for reducing power consumption consists in configuring them in single-ended instead of in bridge configuration, whenever it is possible to do so. In fact, these amplifiers dissipate more power in bridge configuration than in single-ended configuration as long as the amplitude of the output signal remains smaller than the positive supply voltage. Unfortunately, it is not possible to use single-ended class AB amplifiers if the output surpasses this voltage because the output signal would be severely distorted by clipping.
Techniques for automatically switching from one configuration to the other in function of the monitored level of the signal are implemented in systems disclosed in U.S. Pat. No. 5,194,821, U.S. Pat. No. 5,365,188 and U.S. Pat. No. 5,654,688, all in the name of the same Applicant.
U.S. Pat. No. 5,194,821 discloses a bridge amplifier using a positive and a negative supply voltage sources. The amplifier may function in single-ended or in differential or bridge output configuration, depending on the level of the output signal. A comparator changes the output circuitry configuration of the amplifier from a bridge configuration to a single-ended configuration or vice versa by closing or opening configuring switches, when the output signal becomes smaller than or greater than a certain threshold voltage.
U.S. Pat. No. 5,365,188 and U.S. Pat. No. 5,654,688 disclose a single supply, dual bridge power amplifier employing a window comparator for sensing the level of input signals fed to the amplifier and driving switches that by coordinately changing state configure the amplifier in either a bridge or in a single-ended configuration.
Recently, power amplifiers with digital input and analog output have been proposed. As shown in FIG. 1, they are substantially composed of a digital block receiving digital input signals and generating corresponding output signals for driving the channels of the power stage. Even for these amplifiers it is possible to reduce power consumption, according to the techniques disclosed in the mentioned patents, using dedicated comparators contained in the power stage that configure the channels in single-ended or bridge configuration depending on the level of the output signals.
The above mentioned power amplifiers show good performances in terms of power consumption and low distortion of the output signals when they have a relatively low frequency, but it has been noticed that distortion increases progressively as the frequency of the processed signal increases.